while True:
    print("欢迎使用用户检测系统")
    user = [('root','123456'),('admin','admin')]
    username = input("请输入用户名:")
    passwd = input("请输入密码：")
    if (username,passwd) in user:
        print("登录成功")
    else:
        print("登录失败")
    str = input("请输入quit退出或者任意值继续:")
    if str == "quit":
        break
